Wiring SIM800L with Arduino (nano/uno) VDD+VCC?

I'm trying to use a SIM800L V2 module but I'm not 100% sure of the wiring to do.
The module: https://www.robotics.org.za/SIM800L-V2

What I'm not sure about is the connector offers two set of pins:

  • Power: VCC + GND
  • UART (serial communication): VDD, GND, TX, RX

I actually power it through VCC+GND and connect only TX/RX.

Should I also connect VDD to 3.3v on Arduino and link the 2 GND ?
Should I adapt signals with mosfets (its gona be a project by itself to exchange data)?

The internal schematic of the module (https://www.electrodragon.com/w/images/4/4a/Sim800L_schematic.jpg) shows the TX/RX has pull up resistor to VDD and when I plug the module on Arduino (nano) the voltage on serial pins (I use D6/D7 or D10/D11) drop of about 2 to 2.5 volts which could explain why I can't issue any AT command).

Why that question ?
Once the module is properly powered, to accept current peaks (simple capacitor helps), and some successful usage, the module(s) stopped to connect to the network. To debug that, I discovered I can't even communicate (AT commands) with the module and looking at the communication I saw the signal was not between 0 and 5v (when no device connected) but from 0v to about 2.5v. This may not help the Arduino to understand the signal on its RX pin...

why the device is not connecting anymore to the network it connected before, is not the question, but feel free to share experiences... :wink:

  • device seems properly powered (dedicated 3A/5v power + capacitor)
  • even powered on its own without anything else around it doesn't connect
  • SIM card is ok (work in cell phone without pin code)


Finally found the specification (https://cdn-shop.adafruit.com/datasheets/sim800h_hardware_design_v1.00.pdf).

I should adapt signals (page 32). Now the game is to find the right mosfet...